Molecular sieves have been well known in the art for many years. In general, these may be of two types: the zeolitic type, which comprises crystalline aluminosilicate molecular sieves, and other molecular sieves which are not of this crystalline aluminosilicate composition.
The naturally occurring and synthetic analogues of the zeolites include over a hundred compositions. Zeolites are, by definition, tectosilicates, which means that their framework comprises tridimensional structures made of SiO.sub.4.sup.-4 and AlO.sub.4.sup.-5 tetrahedra which share vertices with oxygen atoms. The zeolites can be characterized as having porous structures with openings of uniform dimensions, ion-exchange capacity; and the capacity to reversibly adsorb and desorb molecules within the cavities present in the crystals via the pore openings. These pore openings are defined by the linkage of TO.sub.4 tetrahedra, wherein T represents either silicon or aluminum atoms.
Zeolites are synthesized in general by hydrothermal methods from reactive components in closed systems. A large inventory of empirical data on synthesis compositions and conditions leading to the formation of given zeolites is available in the literature. Practice has shown that a wide variety of zeolitic products can be obtained from the same starting composition, depending on the raw materials, mixing methods, and crystallization procedures employed.
Other crystalline molecular sieves, which are not zeolites, are also well-known. A silica polymorph, which exhibits molecular sieve properties but lacks exchangeable cations, is described in U.S. Pat. No. 4,061,712. This polymorph is known as Silicalite. Crystalline aluminumphosphates with molecular sieve properties representing a new class of adsorbents are described in U.S. Pat. No. 4,310,440. The properties of these aluminumphosphates are somewhat analogous to zeolitie molecular sieves and, therefore, these are useful as catalyst bases or catalysts in various chemical reactions. U.S. Pat. No. 4,440,871 and European Patent Application 0146389 describe crystalline silicoaluminumphosphates with molecular sieve, ion-exchange and catalytic properties analogous to zeolites and/or aluminumphosphate molecular sieves.
Molecular sieve, ion-exchange and catalytic properties, akin to those of zeolites, are also found in certain metallosilicates, in which elements such as beryllium, boron, gallium, iron, titanium, and phosphorus are used as substitutes for the silicon or aluminum. These are described in E. Moretti et el., "Zeolite Synthesis in the Presence of Organic Components," Chimica e Industria, 67 (1985) 21-34.
However, all of the crystalline materials described above are known to have free apertures ranging from about 2.1 to about 7.4 Angstroms. The maximum apertures appear to be defined by rings of twelve TO.sub.4 tetrahedra. To date, while there have been reports of the synthesis of non-zeolitic molecular sieve compositions having larger apertures, these reports have not been substantiated. For example, U.S. Pat. No. 4,310,440 describes an aluminumphosphate composition referred to as ALPO4-8 (see example 62-A of that patent) which is reported to significantly adsorb perfluorotributylamine, PFTBA [(C.sub.4 F.sub.9).sub.3 N)]. PFTBA is known to have a kinetic diameter of about 10 A. See R. M. Barter, Zeolites and Clay Minerals (1978) 7. A similar claim is made for the zeolite referred to as AG-4 in British Patent 1,394,163. However, neither of these references provides sufficient data to determine definitively whether the PFTBA molecules are adsorbed in the micropores themselves, in capillary pores between the crystalline particles, or perhaps in impurities that are either crystalline or amorphous.
Other materials reported to have large pores are Z-21 described in U.S. Pat. No. 3,567,372, and zeolite N, similar to Z-21, described in U.S. Pat. No. 3,414,602. More recently, Russian workers have claimed a large pore zeolite based on X-ray powder diffraction data. (See "Neorganicheskie Materialy," Izvestiya Akademii Nauk SSSR 17, 6 (June 1981) 1018-1021.)